1. Field of the Invention
The present invention relates to an etching liquid for controlling a silicon wafer surface shape and a method for manufacturing a silicon wafer using the same for reducing loads of a both-side simultaneous polishing process, and attaining both a high flatness degree and the reduction of surface roughness.
2. Description of the Related Art
Generally, the manufacturing processes of a semiconductor silicon wafer include processes of chamfering, mechanical polishing (lapping), etching, mirror grinding (polishing) and cleaning on a wafer that is cut and sliced from a silicon single crystal ingot pulled up, and the wafer is manufactured into a wafer having a highly precise flatness degree. A silicon wafer that goes through mechanical manufacturing processes including block cutting, outer diameter grinding, slicing, lapping and the like has a damaged layer, i.e., a work-affected layer on the surface thereof. The work-affected layer induces crystal defects such as a slip dislocation and the like in the device manufacturing processes, and decreases the mechanical strength of the wafer, and causes adverse effects on the electrical characteristics thereof, and accordingly such defects must be removed completely.
An etching process is performed so as to remove this work-affected layer. In the etching process, either an acid etching method or an alkali etching method is employed. In this etching process, a plurality of wafers are dipped into an etching bath containing an etching liquid, thereby the work-affected layer is chemically removed.
The acid etching has advantages that there is no selective etching property to a silicon wafer, and the surface roughness is small, therefore, the micro shape precision is improved, and the etching efficiency is high. As the etching liquid of this acid etching, an etching liquid of three components obtained by thinning mixed acid of hydrofluoric acid (HF) and nitric acid (HNO3) by water (H2O) or acetic acid (CH3COOH) is mainly employed. It is considered that the acid etching has the above advantages because etching progresses on the basis of diffusion controlling conditions by the above etching liquid, and under the diffusion controlling conditions, the reaction speed does not depend upon the crystal orientation of the crystal surface, crystal defects and the like, and the diffusion on the crystal surface has a main effect. However, in this acid etching, although the work-affected layer can be etched while improving the surface roughness of a silicon wafer, as the acid etching progresses, the outer circumferential portion of the wafer becomes dull, and the flatness degree as micro shape precision obtained by lapping is deteriorated, which causes mm-order concaves and convexes called swells or peels on the etched surface. Further, the cost of the chemical liquid is high, and it is difficult to control and maintain the composition of the etching liquid, which has been a problem in the prior art.
The alkali etching has advantages that the flatness degree is excellent and the macro shape precision is improved, and there is little metal contamination, and there is no problem of harmful secondary products like NOx in the acid etching or no danger in handling thereof. As the etching liquid of this alkali etching, KOH and NaOH are employed. It is considered that the alkali etching has the above advantages because this etching progresses basically on the basis of diffusion controlling conditions. However, in the alkali etching, although the work-affected layer can be etched while maintaining the flatness degree of a silicon wafer, there occur facets whose partial depth is several μm, and whose size is several to several ten μm (hereinafter, referred to as facets) that deteriorates the wafer surface roughness, which has been another problem in the prior art.
As a method to solve the problems in the alkali etching, there is disclosed an etching method of a silicon wafer wherein an etching liquid obtained by adding 0.01 to 0.2 weight percentage of hydrogen peroxide to 100 weight percentage of a caustic soda (sodium hydroxide) aqueous solution is employed (for example, see Patent Document 1) According to the etching method disclosed in the above Patent Document, by adding hydrogen peroxide to a caustic soda aqueous solution at a specified percentage, nonconformities arising in the alkali etching by the caustic soda aqueous solution are solved. Specificially, in comparison with the etching using a NaOH aqueous solution, the size of an etched pit on the lowerside surface of a silicon wafer is made finer. Furthermore, the occurrence of micro etched pits on the lowerside surface of the silicon wafer is restricted, and the desired etching speed can be adjusted easily and in a wide area, and furthermore, the etching speed is increased.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H07-37871 (claims 1 to 4, paragraph [0021])
However, in the conventional methods including the method disclosed in the above Patent Document, a wafer after the etching process is sent to a both-side simultaneous polishing process and a one-side polishing process wherein the surface thereof is processed into a mirror surface. However, in the upperside and lowerside surfaces of the silicon wafer after the etching process, the wafer flatness degree obtained at completion of the planarizing process is not maintained. Furthermore, a desired wafer surface roughness is not obtained yet, and accordingly, in order to improve the wafer flatness degree and the wafer surface roughness, it is necessary to move or remove large amounts of grinding residue in the both-side simultaneous polishing process and the one-side polishing process. As a result, this creates additional time and energy loads on the both-side simultaneous polishing process and the one-side polishing process.